1. Field of the Invention
The present invention relates to a digital trunk which is used in a digital electronic exchange to perform in-band (voice frequency band) line signal control, and more particularly, to a digital trunk including a local loopback test capability for an in-band line signal circuit by on-line call processing.
2. Description of the Related Art
Generally, digital electronic exchange, using an in-band line signaling system as one of line signaling systems, is provided with a digital trunk for controlling the in-band line signal, and the loopback test of the in-band line signal circuit is suitably to be performed to maintain normality of the in-band line signal circuit.
A technique on the loopback test of the in-band line signal circuit such as this is mentioned, for example, in Japanese Patent Application Laid-Open No. Hei 5-95410.
FIG. 1 is a block diagram showing a technology of the loopback test of the in-band line signal disclosed in the Japanese Patent Application Laid-Open No. Hei 5-95410.
The digital electronic exchange is provided with a central control 101, a time division switch 102, a multiplexer/demultiplexer 103 and a digital trunk 104, and the multiplexer/demultiplexer includes a multiplex/demultiplex circuit 131, an in-band signal transmitter 132, an in-band signal receiver 133 and a control unit 134.
In an ordinary call processing, the in-band line signal is received by the in-band signal receiver 133 through the digital trunk 104 and the multiplex/demultiplex circuit 131 which multiplexes all incoming signals. The in-band signal receiver 133 transfers the received in-band line signal to the central control 101 via the time division switch 102 for call processing purposes, and also transfers to the control unit 134 for detecting idle condition or busy condition of each time slot and classifying each signal. When the loopback test is performed, the control unit 134 detects idle time slot, and instructs the multiplex/demultiplex circuit through control line 137 to connect an incoming line 135 and an outgoing line 136 corresponding to the idle time slot having been detected. Then, the in-band signal is transmitted from the in-band signal transmitter 132 and received by the in-band signal receiver via loopback established in the multiplex/demultiplex circuit 131, and normality of the signal is confirmed.
Hitherto, in the case where the local loopback test of a digital circuit with the in-band line signal is performed in the exchange, there is another method where the transmission-reception test of the in-band line signal is performed with a transmission time slot (TS) to a circuit, as it is, returned back to the same TS on the reception side, because a line signal channel for call control and a speech channel use the same circuit, and the normality of the signal channel and speech channel is confirmed at the same time.
Furthermore, when performing the local loopback test of the digital circuit provided with in-band line signals, there is also a method where TS (not being occupied) in an idle state is detected and a transmission-reception looping test is made only for an in-band line signal which corresponds to the aforementioned TS (the transmission TS number and the reception TS number are the same).
The aforementioned loopback test, incidentally, has the following problems.
First, in the conventional loopback test, the TS on the transmission side, as it is, is simply returned to the same TS on the reception side at the time of the loopback test and transmission and reception of the signal is only confirmed. Therefore, there is the problem that the call processing sequence with the opposite exchange cannot be simulated and that the normality of a part of control hardware involved itself can only be verified but the normality of other relating parts, such as the operation of an upper hierarchy control device, on-line software and the office data, cannot be verified.
Furthermore, at the time of the loopback test, the tested circuit cannot be used for ordinary call. Therefore, it is problem that the number of circuits provided for the opposite exchange is decreased.